The invention lies in the field of semiconductor technology and relates to a method for monitoring a semiconductor fabrication process for processing a substrate.
A multiplicity of fabrication processes are used in fabricating and processing semiconductor substrates to form integrated semiconductor circuits therein. Deposition processes and etching processes for patterning layers that are applied on a substrate are mentioned as examples. These fabrication processes must be monitored, in principle, since, because of their complexity, unnoticed disturbances or poorly adapted process conditions can lead to defectively fabricated semiconductor circuits. To be able to efficiently carry out this monitoring, there is generally a desire to characterize the fabrication process through real-time analysis of specific measurement quantities that are determined during the fabrication process to thereby be able to make a regulating intervention, if appropriate.
Possible methods for monitoring fabrication processes are disclosed, for example, in U.S. Pat. No. 5,877,032, which describes a method for determining the end point of a plasma etching process, in which the detected optical emission of the plasma is used for determining the end point. The background to this approach is the fact that, during etching processes, a layer situated on a substrate is etched through and the underlying substrate is uncovered in the process. The interaction between the etching gas and the uncovered substrate can be demonstrated spectroscopically as a change in the emission spectrum of the plasma. In accordance with U.S. Pat. No. 5,877,032, this change is compared with a multiplicity of predetermined reference curves and the end point of the plasma etching process is inferred from the comparison.
U.S. Pat. No. 5,739,051 likewise discloses a method for determining the end point of a plasma etching process, in which the optical emission of the plasma is likewise used for determining the end point. Emission lines that are characteristic of the interaction between the etching gas and the substrate are used for the assessment.
However, very often it is difficult to extract the measurement quantity that is characteristic of the etching process from the multiplicity of available spectra or else from other measurement quantities. Therefore, U.S. Pat. No. 5,658,423 proposes a method based on so-called principal component analysis, in which the temporal development of the entire emission spectrum from about 240 to 600 nanometers is used for the end point analysis. Using principal component analysis, the volume of data obtained is reduced to a few so-called base patterns and the temporal development thereof is used for detecting the end point. As a result of this, the detection of the end point is no longer based on the assessment of a single emission wavelength, but on the change in the entire available spectrum. In principle, however, this approach also requires that reference values be provided for comparison with the currently measured measurement quantities.
In U.S. Pat. No. 5,737,496, an attempt is made to avoid the last-mentioned problem, in particular, by using a neural network. The neural network is trained using a multiplicity of determined measurement quantities, so that it can subsequently be used for decision-making with regard to the end point identification. It has been shown, however, that neural networks often learn incorrect signals and patterns, so that an incorrect interpretation can occur. Erroneous training of the neural network arises, for example, through a change in the emission spectra because of aging phenomena of the sensors or because of chamber contamination that occurs. Therefore, U.S. Pat. No. 5,864,773 proposes a so-called virtual sensor system, in which these changes are taken into account before the measurement quantities are actually assessed. As a result, the intention is to produce a virtual sensor that is free of chamber-specific or process-specific fault effects. Since it is necessary to have recourse to the operating personnel""s experiences in this case, too, unexpectedly occurring faults and changes cannot automatically be taken into account.
It is accordingly an object of the invention to provide a method for monitoring a semiconductor fabrication process for processing a substrate which overcomes the above-mentioned disadvantages of the prior art methods of this general type.
In particular, it is an object of the invention to provide a method for monitoring a semiconductor fabrication process for processing a substrate which enables the fabrication process to be monitored reliably and in a manner that is, as far as possible, free from faults. Even more particularly, the method serves for determining the end point of the fabrication process.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for monitoring a plasma process that includes steps of:
using a first model to determine an end point of a first plasma process that is performed in a plasma;
defining the first model with an algorithm, with a termination criterion, and with at least one predetermined measurement quantity that can be determined during the first plasma process and that is based on an intensity of at least one predetermined emission wavelength of the plasma;
configuring the algorithm such that when the algorithm is applied to the predetermined measurement quantity that is determined, the algorithm provides a decision quantity which, upon comparison with the termination criterion, serves for determining the end point of the first plasma process;
performing the first plasma process by using a plasma-excited gas in a plasma chamber, by introducing a substrate, which will be treated, into the plasma chamber, and by allowing the substrate to interact with the plasma-excited gas in the plasma chamber;
during the first plasma process, determining the predetermined measurement quantity for the first model to thereby obtain a measured quantity;
applying the algorithm of the first model to the measured quantity and determining the decision quantity;
comparing the decision quantity with the termination criterion prescribed by the first model and terminating the first plasma process when the termination criterion is met;
using a second model for comparatively determining the end point of the first plasma process;
defining the second model with an algorithm, with a termination criterion, and with at least one predetermined measurement quantity that can be determined during the first plasma process and that is based on an intensity of at least one predetermined emission wavelength of the plasma;
using an additional monitoring function for continuously assessing the first model and the second model;
if the end point that was determined with the second model has a higher significance than the end point that was determined with the first model, then using the second model to determine an end point of a second plasma process succeeding the first plasma process;
measuring intensities of a plurality of emission wavelengths of the plasma during the first plasma process;
using the intensities of the plurality of the emission wavelengths as measurement quantities and continuously storing the measurement quantities in a data processing system;
also using the data processing system for identifying the end point of the first plasma etching process;
performing the first plasma process as a plasma etching process and providing the plasma-excited gas as a dry etching gas that etches at least parts of the substrate;
providing the substrate with an insulating layer; and
etching contact holes using the plasma etching process.
In accordance with an added feature of the invention, the method includes: using the second model during the second plasma process; performing the second plasma process in a plasma; using a third model for comparatively determining the end point of the second plasma process; defining the third model with an algorithm, with a termination criterion, and with at least one predetermined measurement quantity that can be determined during the second plasma process and that is based on an intensity of at least one predetermined emission wavelength of the plasma of the second plasma process; and if the end point that was determined with the third model has a higher significance than the end point that was determined with the second model, then using third model for determining an end point of third plasma processes.
In accordance with an additional feature of the invention, the method includes: determining a significance of the end point that was determined with the first model and determining a significance of the end point that was determined with the second model by comparing a temporal development of the measurement quantity based on the predetermined emission wavelength of the first model with a temporal development of the measurement quantity based on the predetermined emission wavelength of the second model.
In accordance with another feature of the invention, the method includes: determining a significance of the end point that was determined with the first model and determining a significance of the end point that was determined with the second model by comparing the decision quantity that was determined by the algorithm of the first model with a decision quantity that is determined by the algorithm of the second model.
In accordance with a further feature of the invention, the method includes: comparing the decision quantity that was determined by the algorithm of the first model with the termination criteria of the first model to obtain a first result; comparing the decision quantity that was determined by the algorithm of the second model with the termination criteria of the second model to obtain a second result; and determining the significance of the end point that was determined with the first model and determining the significance of the end point that was determined with the second model by comparing the first result with the second result.
In accordance with a further added feature of the invention, the method includes: basing the measurement quantity of the first model and the measurement quantity of the second model on a common emission wavelength; and determining a measure of contamination of the plasma chamber by comparing the measurement quantity of the first model and the measurement quantity of the second model.
In accordance with a further additional feature of the invention, the method includes making the insulation layer from silicon oxide.
In accordance with yet an added feature of the invention, the method includes etching the contact holes to have different depths in the insulation layer.
In accordance with yet an additional feature of the invention, the method includes: using a rotating plasma during the first plasma process; and using a rotating plasma during the second plasma process.
In accordance with an added feature of the invention, the method includes obtaining the measured quantity by forming a mean value over a predetermined period of time.
In accordance with an additional feature of the invention, the method includes: using a rotating plasma during the first plasma process; and setting the predetermined period of time to correspond to at least one circulation period of the rotating plasma.
In accordance with another feature of the invention, the method includes: using the algorithm of the first model to determine a position of a local maximum, a local gradient, or a point of inflection of a curve representing a temporal development of the measurement quantity of the first model to thereby yield the decision quantity; and using the algorithm of the second model to determine a position of a local maximum, a local gradient, or a point of inflection of a curve representing a temporal development of the measurement quantity of the second model to thereby yield a decision quantity.
In accordance with a further feature of the invention, the first model differs from the second model at least in terms of the predetermined measurement quantity of the first model or the predetermined algorithm of the first model.
In accordance with a further added feature of the invention, the method includes using stored measurement quantities from preceding fabrication processes for determining a significance of the first model and of the second model.
The invention provides a method for monitoring a fabrication process for processing a substrate in a semiconductor fabrication, which has the steps of:
prescribing a first model for determining the end point of the fabrication process; the first model being defined by an algorithm, by at least one predetermined measurement quantity which can be determined during the fabrication process, and by a termination criterion; the algorithm, when applied to the measurement quantity determined, yields a decision quantity which, upon comparison with the termination criterion, serves for determining the end point of the fabrication process;
carrying out the fabrication process in a chamber suitable for this purpose; and introducing a substrate to be treated into the chamber and processing the substrate in the chamber;
determining the measurement quantity that is predetermined by the first model during the fabrication process;
applying the algorithm of the first model to the measurement quantity that is determined and determining the decision quantity;
comparing the decision quantity with the termination criterion that is prescribed by the first model; and ending the fabrication process when the criterion is met;
using a second model for comparatively determining the end point of the fabrication process; the second model being likewise defined by an algorithm, by at least one predetermined measurement quantity that can be determined during the fabrication process, and by a termination criterion; the second model being used for determining the end point of a further fabrication process that succeeds the fabrication process, provided that the end point is determined by the second model with a higher significance than with the first model.
In the method according to the invention, first a first model is prescribed. In this case, this model is determined, in particular, by an algorithm, by the selection of at least one predetermined measurement quantity that can be determined during the fabrication process, and by a termination criterion. The measurement quantity may be, for example, the intensity of the optical emission of the plasma at a wavelength that is predetermined by the model, the pressure, the temperature and other quantities that can be determined during the fabrication process. The measurement quantity is preferably determined using a sensor that is provided on the chamber and the measurement quantity is fed to a data processing system.
The algorithm prescribed by the model is applied to the measurement quantity that is determined during the fabrication process and in this case yields a decision quantity. The predetermined algorithm is generally assigned a termination criterion that is characteristic of the algorithm, which termination criterion is compared with the determined decision quantity. The end point of the fabrication process is determined from this comparison.
Since the meaningfulness of the measurement quantity or the decision quantity that is determined from the measurement quantity by the algorithm can be impaired because of chamber contaminants, the invention proposes using a second model for comparatively determining the end point of this fabrication process. The second model is likewise defined by an algorithm, by at least one predetermined measurement quantity that can be determined during the fabrication process, and by a termination criterion. The first and second models generally differ at least with regard to the measurement quantity selected or with regard to the algorithm used. However, it is also possible for the second model to use a different measurement quantity and a different algorithm than the first model.
The second model is then likewise used for determining the end point of the fabrication process. The significance of the meaningfulness of the two models are compared with one another. This can be done, for example, by directly comparing the significance of the respective decision quantities of the first and second models with one another. Another possibility is to access the determined measurement quantities with regard to their meaningfulness when the respective algorithms are used. Thus, the intention is, for example, to check whether the measurement quantities are excessively noisy or whether other events occurring during the measurement unfavorably alter the measurement quantities.
If the first and second models differ only with regard to the predetermined measurement quantity, the significance of the two models can be assessed, for example, by directly comparing the two measurement quantities. If the two measurement quantities used represent different emission wavelengths, then it is possible, for example, to use the magnitude of the signal swing when reaching the end point as a measure for determining the significance.
In order to minimize the uncertainties that occur in the prior art in the prediction of the measurement quantities that currently will be measured, according to the invention, first the first model is still used to determine the end point, but the second model is used for a subsequent fabrication process, provided that the second model determines the end point with a higher significance. This enables reliable determination of the end point over many fabrication processes while taking account of changes that occur in the measurement quantities because of chamber contamination, drift of sensors, and also because of changes that stem from different fabrication processes that are carried out in the same chamber. As a result, it is possible, for example, to use the chamber for longer without interim costly cleaning, or to carry out different fabrication processes in one chamber in a targeted manner so that changes because of these different fabrication processes, in part, mutually compensate for one another. This can be observed, for example, when etching processes using different etching gases are carried out in one and the same chamber such that the contamination of one etching process is at least partly removed by the etching gases of the other etching process.
A higher significance in the determination of the end point is manifested e.g. in the decision quantity, upon comparison with the termination criterion, leading to a clearer result than in the case of another model. Another possibility for determining the significance consists in comparing the curve profiles of the measurement quantities of the individual models and assigning a higher significance to that measurement quantity and hence to that assigned model whose curve profile is the most similar to a predetermined mode curve.
However, it is also within the scope of the invention for a plurality of models to be used simultaneously for determining the end point, and for the model with the greatest meaningfulness to be used. One essential advantage of the invention consists in using measurement quantities independently of their correlation with process specifics. Thus, by way of example, in the case of an emission spectrum of a plasma, only a few emission lines are characteristic of the interaction between the etching gas and the substrate. It is customary, therefore, for precisely these lines to be selected for end point determination. If these lines can no longer be used for assessing the end point because of chamber contamination, the chamber has hitherto had to be cleaned. Consequently, the preselected lines could only be used under xe2x80x9cgoodxe2x80x9d process conditions.
Using the inventive method, it is now possible to select further emission lines that are not even characteristic or not very characteristic of the interaction between the etching gas and the substrate, provided that they yield a useable and reliable end point signal when the algorithms prescribed by the models are used. Thus, in principle, all measurement quantities are available and can be used for determining the end point. In this case, the individual measurement quantities that are available can be tested during the fabrication process and can be processed with the algorithms in order to ascertain whether a suitable end point signal can be generated from them.
If the second model is used for determining the end point in a subsequent fabrication process, furthermore a third model is used for comparatively determining the end point and the third model is used, if appropriate, provided that it determines the end point with a higher significance.
Preferably, the measurement quantities determined are stored and are thus also available for comparative checking. Using these stored measurement quantities, the individual algorithms of the models can be tested and the decision quantities formed in the process can be compared with regard to their meaningfulness. From this comparison it is then possible to determine in each case the xe2x80x9cbestxe2x80x9d measurement quantity and the xe2x80x9cbestxe2x80x9d model for determining the end point for further fabrication processes.
In contrast to the previously known methods, the inventive method thus contains an additional monitoring function that serves for assessing the models. This ensures that a reliable model for end point identification is always provided for the respective fabrication process. This model takes account of the fabrication-specific changes that have occurred previously. As a result, therefore, by selecting the respectively suitable model, despite the occurrence of chamber contamination and other generally undesirable changes, the end point of the fabrication process is determined reliably and with the smallest possible fluctuations. The uncertainties that usually occur in comparing measurement quantities with fixedly predetermined reference values are precluded to the greatest possible extent when using the inventive method.
Suitable algorithms for determining the decision quantity from the measurement quantities determined are, for example, the determination of a local maximum, the local or temporal gradient or the point of inflection of the temporal development of the measurement quantity. The measurement quantities can be stored, for example, in a manner dependent on their temporal development and are thus available for the testing of the algorithms.
However, it is also possible to use a multiplicity of measurement quantities and to apply the algorithm to them. If appropriate, data reduction is effected at the same time. If the measurement quantities represent the emission spectrum of a plasma in a predetermined wavelength range, the so-called principal component analysis can also be used as an algorithm. The analysis determines base patterns representing the spectrum and also the temporal development thereof. In particular the temporal development of individual base patterns can be used as a decision quantity.
The invention furthermore proposes a method for monitoring a plasma process for processing a substrate in semiconductor fabrication having the steps of:
using a first model for determining the end point of the plasma process; the first model being defined by an algorithm, by at least one predetermined measurement quantity that can be determined during the plasma process and that is based on the intensity of at least one predetermined emission wavelength of the plasma, and by a termination criterion; the algorithm, when applied to the measurement quantity determined, yields a decision quantity which, upon comparison with the termination criterion, serves for determining the end point of the plasma process;
carrying out the plasma process using a plasma-excited gas in a plasma chamber; a substrate to be treated is introduced into the plasma chamber and interacts there with the plasma-excited gas;
determining the measurement quantity predetermined by the first model during the plasma process;
applying the algorithm of the first model to the measurement quantity determined, and determining the decision quantity;
comparing the decision quantity with the termination criterion prescribed by the first model; the plasma process being terminated when the termination criterion is met;
using a second model for comparatively determining the end point of the plasma process, which is likewise defined by an algorithm, by at least one predetermined measurement quantity that can be determined during the plasma process and that is based on the intensity of at least one predetermined emission wavelength of the plasma, and by a termination criterion; the second model being used for determining the end point of a further plasma process succeeding the plasma process, provided that the end point was determined by the second model with higher significance than with the first model.
In plasma processes, in particular, there is a need to reliably determine the end point since undesirable incipient etching of the substrate can otherwise occur. This arises, for example, in so-called plasma etching processes in which a plasma-excited gas (dry etching gas) interacts with the substrate. The substrate is usually covered with a masking layer (photomask), so that only the uncovered regions of the substrate come into contact with the dry etching gas and the substrate is actually only etched there. However, layers located deeper in the substrate should not be attacked by the dry etching gas, so that the etching process must be stopped when the substrate is etched through and the layers are reached.
A suitable measurement quantity in plasma etching processes is, in particular, the emission spectrum of the plasma, since a multiplicity of emission wavelengths are available here.
The inventive method is preferably used when etching contact holes into an insulation layer that is situated on the substrate. In this case, the insulation layer is preferably composed of silicon oxide. In the course of etching contact holes in the insulation layer, it is possible at the same time, or else afterward, to produce further structures in the insulation layer, which are finally filled with a conductive material, for example. It is thus possible to fabricate e.g. wiring planes in an integrated semiconductor circuit using so-called demasking technology.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for monitoring a fabrication process for processing a substrate in semiconductor fabrication, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.